A14B-01
Ozone Recovery in the Upper and Lower Stratosphere: Separating the Influence of Changes in Ozone Depleting Substances from that of Greenhouse Gases
As ozone-depleting substances are slowly removed from the stratosphere, the ozone layer should begin to recover. Detection of the initial stages of recovery has been the topic of a number of recent papers. An important aspect of recovery is the attribution of changes to the removal of ozone-depleting substances. The attribution is complicated by expected changes in stratospheric ozone due to increasing greenhouse gases. We use data from SBUV measurements on Nimbus 7 and the NOAA series of satellites to make a first attempt at observing the different expected time signatures from ozone-depleting substances and greenhouse gases. Ozone depleting substances, as represented by the Equivalent Effective Stratospheric Chlorine (EESC) have now peaked and begun their slow decline. Greenhouse gases, such as carbon dioxide, continue to increase nearly linearly. We examine the data in the upper stratosphere (~1-10 hPa) where the greenhouse gas signature is expected to be nearly constant with latitude, and the lower stratosphere (~25-64 hPa) where the greenhouse gas signature is expected to change sign from ozone decreases in the tropics to ozone increases in the mid to high latitudes. Our goal is to use data to confirm the response of ozone to changes in ozone-depleting substances and to eventually detect the response of ozone to greenhouse gases.
A14B-02
The advance of ozone recovery induced by greenhouse gas increases
The most significant effect of climate change on ozone has been predicted for the polar regions, suggesting that for high concentrations of greenhouse gases, an ozone hole could occur in the Arctic, similar to that in the Antarctic. With future projections of chlorine and bromine amounts in the atmosphere, calculations now simulate steady ozone recovery over the next few decades. Here we report that rather than slow down the rate of ozone recovery, the effect of greenhouse gas increases is to advance recovery by several decades. The simulations here show that the extent to which ozone will recover in the future depends on several competing and enhancing factors which operate over multi-decadal time scales. The effect of increasing greenhouse gas amounts is known to warm the lower atmosphere and cool the upper atmosphere, which because of the temperature dependence of the reaction rates, generally increases ozone, except in the polar regions. Climate change also induces changes in atmospheric circulation. Over several decades, our results show that ozone transport generally dominates chemical changes in the northern hemisphere with the reverse in the southern hemisphere. In particular, in the Arctic, ozone recovery is advanced because the increased circulation and associated adiabatic warming increases transport and decreases chemical loss compared with early studies.
A14B-03
Strengthening of the Brewer-Dobson Circulation and Its Impact on Ozone Distributions in the Post-CFC Era: a Coupled Chemistry-Climate Model Study
The stratospheric ozone layer is expected to recover to pre-1980 levels in the middle of the 21st century with the projected decline of the stratospheric halogen loading. Ozone recovery is also strongly dependant on transport and temperature, which are significantly different between the middle 21st century and the 1980s due to greenhouse gas increases. This paper investigates changes in transport and their impacts on ozone distributions in the post-CFC era using simulations from the Goddard Earth Observing System Coupled Chemistry-Climate Model. Model simulations of the recent past and the 21st century with projected greenhouse gas increases under the IPCC A1B scenario are analyzed. Model simulations show an enhancement of the Brewer-Dobson circulation with a trend of 3%/decade in tropical upwelling. The accelerated Brewer-Dobson circulation increases the mean ozone transport, resulting in less ozone in the tropics and more ozone in the extratropics. Model results reveal that, in the 2060s when the stratospheric halogen loading recovers to 1980 values, ozone changes below 30 km can be mainly attributed to the enhanced mean advective transport. The total column ozone changes, however, are determined by the combined effects of a stronger mean transport and slower ozone loss reactions arising from upper stratosphere cooling. In the tropics, enhanced upwelling outweighs reduced ozone photochemical losses, and the model projects that tropical total column ozone does not recovery to 1980 values. In the extratropics, enhanced downwelling, combined with ozone increase above 30 km, causes a "super recovery" of ozone. We will also discuss inter-hemispheric differences in the changes of the transport and the contraction of the tropical upwelling.
A14B-04
Relationships between climate change, natural variability, and stratospheric ozone in the Arctic winter and spring
Observations of the stratosphere suggest that changes in temperature are caused by ozone loss and by greenhouse gas increases, but that regional trends can be dominated by dynamical variability. This is particularly true of the Arctic region, where year-to-year variations in planetary wave forcing (and possibly other factors) appear to strongly modulate the possibility for chemical ozone loss. This study will examine temperature and ozone change between about 1960 and 2008 in the Arctic using observations, meteorological analyses, and the Goddard Earth Observing System Chemistry-Climate Model (GEOS CCM). Analysis of observations will re-examine the suggestion of Pawson and Naujokat (JGR, 1999) that there may be a tendency for dynamically undisturbed winters to become colder between 1960 and 2000. Controlled experiments with the GEOS CCM allow examination of the significance of this apparent trend and isolation of possible mechanisms for it.
A14B-05
How much energy is transferred from the winds to the thermocline on ENSO timescales?
Coupled GCMs exhibit different behavior in the tropical Pacific Ocean, particularly when simulating ENSO or the mean state of the coupled ocean-atmosphere system. Explaining these differences and improving simulations of El Niño remains an essential task for predicting the impacts of Global climate change. Here, we study the dynamics of ENSO in a range of ocean and coupled models in terms of the balance between energy input from the winds and resulting available potential energy (APE) stored in the tropical thermocline. Prior to an El Niño, the wind power decreases which leads to a decrease in the APE and hence an El Niño event. In contrast, La Niña events are preceded by an increase in wind power leading to an increase in the available potential energy. The wind power alters the APE via the buoyancy power, associated with vertical mass fluxes that modify the slope of the isopycnals. This description gives a basin- wide, integral approach ideal for inter-model comparison. We show that only a fraction of the wind power is converted to the buoyancy power. We estimate the efficiency of this conversion, γ, at 50-60% for ocean models and data assimilations, but only at 30-45% for coupled models. Once the energy is delivered to the thermocline, it is subject to small, but not negligible, diffusive dissipation. This dissipation should lead to e-folding damping rates for APE anomalies, α, of the order of 1 years-1 as estimated in ocean models and data assimilations. For many coupled models, however, we estimate significantly higher dissipation rates, of the order of 2 years-1. We discuss how the energy conversion efficiency and subsequent dissipation rates affect the properties of the simulated ENSO and the mean state of the coupled system, as well as decadal changes in the energetics of ENSO.
A14B-06
A Two-Oscillator View of ENSO
By analyzing reanalysis/assimilation data and CGCM simulations, we propose that there may be two different oscillators in ENSO: a central Pacific oscillator which is primarily forced by atmospheric forcing and an eastern Pacific oscillator which is resulted from the air-sea interaction involving the thermocline variation. The warm and cold phases of these two oscillators are not exactly symmetric in their spatial structures. The asymmetric parts result in a net ENSO forcing to the Pacific mean state which gradually shifts the Pacific Walker circulation eastward or westward from its normal location. As the basic state change, the ENSO alternates between the eastern Pacific oscillator and the central Pacific oscillator. These ENSO-mean state interactions give rise to a decadal modulation of ENSO intensity and can explain why super ENSO events occur every 12-to-15 years. Twenty IPCC AR4 CGCMs were analyzed to further examine the two-oscillator view of ENSO and the physical mechanism behind the ENSO-mean state modulation mechanism. The projected climate change are also contrasted between the group of the CGCMs that can capture the ENSO-mean state interaction and the CGCM group that cannot.
A14B-07
The response of the equatorial Pacific Ocean to global warming
Decadal variations of very small amplitude (~0.3C in sea surface temperature) in the tropical Pacific Ocean
have been shown to have powerful impacts on global climate. Future projections from different climate
models do not agree on how this critical feature will change under the influence of anthropogenic forcing. A
number of attempts have been made to resolve this issue by examining trends from the 1880s to the present,
a period of rising atmospheric concentrations of greenhouse gases. The most recent concluded that the
three major data sets disagreed on the trend in the equatorial gradient of sea surface temperature (SST).
Using a corrected version of one of these data sets, and extending the analysis to the seasonal cycle, we
show here that all agree that the equatorial SST gradient strengthened from 1880–2005, especially during
the boreal fall when the gradient is normally strongest. This result appears to favor a theory for future
changes based on ocean dynamics over one based on atmospheric energy constraints. Both theories
incorporate the expectation that, based on ENSO theory, the zonal sea level pressure (SLP) gradient in the
tropical Pacific is coupled to SST and should strengthen along with the SST gradient. We find, however, that
the SLP gradient appears to have weakened over the same time period, though consistent with the SST
seasonal trends, it weakens least in boreal fall. Most of the IPCC AR4 models capture prominent features of
these trends, but they fail to reproduce observed trends in boreal spring.
http://www.ldeo.columbia.edu/~krisk/nature_submitted.pdf
A14B-08
Response of ENSO to global warming: A perspective from the global heat balance
I will review progress that has been made in understanding the role of ENSO in the global heat balance. In
particular, I will highlight the research leading to the view that averaged over the decadal or longer time-
scales, ENSO acts as a basin-scale heat mixer in the tropical Pacific. This heat mixer regulates the long-term
temperature difference between the surface water in the warm-pool and the subsurface water constituting
the equatorial undercurrent. When this temperature difference is externally forced to increase, the level of
ENSO activity increases. Conversely, when this temperature difference is externally forced to decrease, the
level of ENSO activity decreases. The time-mean effect of ENSO is to counteract the effect of external forcing
on this temperature difference. In this view, the level of ENSO activity is controlled not only by tropical
heating, but also by extra-tropical cooling. It suggests that we shall see an elevated level of ENSO activity in
the initial stages of global warming, but a reduced level of ENSO activity (or even a permanent El Nino state)
when global warming is full-blown. I will attempt to explain why projections from the most complicated climate
models actually miss this scenario.
http://www.cdc.noaa.gov/people/dezheng.sun